The Earth is one of the nine planets that orbit around the
sun in elliptical orbits. An orbit is the time it takes for a
planet or other satellite to go around the sun and end up in the
same place it started. The closer a planet is to the sun, the
shorter the time to complete one revolution is. Mercury, for example,
has an orbital period of 88 days while it takes Jupiter about
11.86 years. The period of the Earth's orbit is usually thought
to be 365 days as calendars show. However when averaged it is
actually about 365.25 days, which requires an extra day on the
calendar every four years (causing a leap year).

Contradictions within orbit times are probably caused by the
ages of the sources. However, I believe that the time given by
the NASA website is more right because of NASA specialized experience
of space and because its web pages are constantly being updated
with accurate information. I also found that the year decreases
due to a gradual slowing of the earth's rotation by a rate of
half a second per century. So far through 20 centuries, the year
has slowed down by 10 seconds.

The atomic clock is another accurate source, however it recorded
365.24 days, which is lower than NASA's results by about 8 minutes.
Between these two results, I go with the atomic clock's results
since atomic clocks examine atoms and their age and are accurate
to the quadrillionth of a second. Atomic clocks use oscillations
of atoms instead of pendulums or other mechanical devices and
are thus extremely accurate. Their oscillations are compared to
the atomic system and then a time is calculated.